Polyimides are polymers which form films having excellent chemical, physical and electrical properties. The polyimides which are usually available today are generally solutions of the polyimide precursors in suitable solvents. After application of the coatings, the coatings are dried and cured, forming the insoluble imide structures. Various polyimide precursors are known for this purpose. Customarily, suitable primary diamines are reacted with tetracarboxylic acid dianhydrides to give polyamidocarboxylic acids, as described, for example, in GB-B 898 651, U.S. Pat. No. 3,207,728 or U.S. Pat. No. 3,234,181. Such polyamidocarboxylic acids are soluble in suitable solvents, eg. N-methylpyrrolidone. After application of the coatings, the coatings are dried and cured, forming the insoluble imide structures by elimination of water. The polyimides produced in this way have, if they have been prepared, for example, from bis(4-aminophenyl) oxide and pyromellitic dianhydride, glass transition temperatures of above 400.degree. C. and decomposition temperatures of about 580.degree. C. The polyamidocarboxylic acids are soluble in certain solvents, eg. N-methylpyrrolidone, but have, owing to their molecular structure, a high viscosity and a comparatively low solids content. They are not stable on storage at room temperature and have to be stored at low temperatures, eg. from 0 to 5.degree. C. For some applications, eg. planarization in chip production, their low solids content is not optimal.
A higher solids content can be obtained by means of formulations in which dialkyl polyamidocarboxylates are used. These formulations are described, for example, in EP-A-0 209 670 and have improved stability on storage and planarization, but they are expensive to produce.
Solutions prepared from diamines and tetracarboxylic acid diesters, as described, for example, in DE-A-41 35 056 and DE 41 35 058, represent polyimide precursors which are more favorable in terms of the solids-viscosity ratio. The solutions are stable on storage at room temperature. Appropriate application and curing of the solutions gives polyimide films. At solids contents of 50% by weight and above, these solutions have viscosities which correspond to those of the commercial polyamidocarboxylic acid solutions. During curing, the esterifying alcohols and water are eliminated. Depending on the starting materials used, polyimides having glass transition temperatures around 300.degree. C. and decomposition temperatures around 520.degree. C. are obtained after curing. The poorer thermal properties result from the imperfect polyimide structures formed as a result of curing.